The present invention relates to a steam valve assembly having a main steam-stop valve and a steam control valve, and to a steam turbine plant that has the steam valve assembly.
In a typical conventional steam turbine plant, the steam coming from the boiler is applied to the steam turbine through the steam valve assembly. The steam that has done work in the steam turbine is changed back to water in the condenser. The condensed water is then supplied by the water pump to the boiler. Water is thus circulated in the steam turbine plant. The steam valve assembly has a main-steam stop valve and a steam control valve that is arranged downstream of the main-steam stop valve. The main-steam stop valve can instantaneously stop steam flowing into the steam turbine if abnormality develops in the steam turbine. The steam control valve is configured to control the flow rate at which stream is supplied to the steam turbine.
In the typical conventional steam turbine plant, the main-steam stop valve and the steam control valve are integrated, forming a single unit. Various structures of combinations of the two types of valves have been proposed in the art.
For example, a main-steam stop valve of horizontal type (laid horizontal) and a steam control valve if vertical (standing vertical) type may be combined as disclosed in Japanese Patent Application Laid-Open Publication No. 2006-183582, FIG. 8 (the entire content of which is incorporated herein by reference). In this case, the valve body of the main-steam stop valve is driven by a hydraulic cylinder arranged beside a side cover, via a valve stem that penetrates the side cover and extends horizontally. Further, the valve body of the stream control valve is driven by another hydraulic cylinder arranged above the top cover (or below the bottom cover), via another valve stem that penetrates the top (or bottom) cover and extends vertically.
In some steam valve assemblies hitherto used, both the main-steam stop valve and the steam control valve are vertical (standing vertical types) as disclosed in Japanese Patent Application Laid-Open Publication No. 2006-183582, FIG. 8 and FIG. 12; Japanese Patent Application Laid-Open Publication No. 10-176502, FIG. 1; and Japanese Utility Model Application Laid-Open Publication No. 63-82002 (the entire contents of which are incorporated herein by reference). In such a steam valve assembly, the valve body of either the main-steam stop valve or the steam control valve is driven by a hydraulic cylinder arranged above the casing, via a valve stem that extends vertically and penetrates the casing. On the other hand, the valve body of the other valve is driven by another hydraulic cylinder arranged below the casing, via another valve stem that extends vertically and penetrates the casing.
Of the conventional steam valve assemblies described above, that one which uses a main-steam stop valve of horizontal type (laid horizontal) and a steam control valve of vertical (standing vertical) type are combined is disadvantageous in the following respects, because the valve stem of the horizontal-type (laid horizontal) valve extends in horizontal direction.
(1) Since the valve stem bends due to the weight of the valve body, the valve body at the distal end of the valve stem cannot fully contact the valve seat, possibly failing to block the steam flowing to it. Consequently, the contact between the valve body and the valve seat must be adjusted to compensate for the bend of the valve stem. This requires much skill and long time on the part of the person who assembles the steam valve assembly.
(2) While the horizontal-type (laid horizontal) valve is being removed in order to accomplish periodical inspection of the valve in power plants, no wire ropes cannot be wrapped around the valve until the valve body and valve stem are pulled out in horizontal position, completely from the casing of the steam valve assembly. Further, the center gravity of the valve body and the valve stem cannot be determined while they are being pulled out. Inevitably, the persons who are holding and removing the valve cannot help but take unnatural positions. This is undesirable from a viewpoint of labor safety management. This also decreases the efficiency of the maintenance work.
Not a combination of a valve of horizontal type (laid horizontally) and a valve of vertical type (laid vertically), but a combination of a main-steam stop valve and a steam control valve, both being conventional vertical (standing vertical) type, may be used as a steam valve assembly. In a steam valve apparatus that is a combination of the two conventional vertical type valves, however, any one of the valve stems that slide up and down through the casings of the main-steam stop valve and steam control valve extend toward downstream side of the valve seat. Preferably, most valve apparatuses should have no components that may cause a pressure loss downstream of any valve seat. In a steam valve assembly, which is a combination of the two conventional vertical (standing vertical) type valves, the valve stem of the two steam-stop valve or steam control valve makes an obstacle to the flowing steam when the valve is opened. This inevitably results in a large pressure loss.